Constant infusions of MTX into mice have been conducted in order to provide us further understanding on organ specificity with respect to rate of inhibition and rate of recovery of certain biochemical pathways and cell growth. In vivo exposure to drug has been followed by in vitro cloning of nucleated cells to determine bone viability at different stages of drug exposure and of drug removal. In vitro colony forming ability of normal bone marrow has been tested with combinations of MTX and several rescue agents (calcium leucovorin, thymidine, hypoxanthine, inosine, guanosine and adenosine). In vivo rate of inhibition and rate of recovery of certain biochemical pathways in small intestine, bone marrow and Lewis lung tumor after constant infusions of MTX have been studied. Changes in recovery rates of these biochemical pathways in the three different organs have been studied following administration of a variety of agents as listed above.